1. Field of the Invention
The present invention relates to switching power supplies and more particularly relates to a switching power supply that feeds DC electric power from a DC voltage source to a load via a transformer with an improved power factor.
2. Description of the Prior Art
FIG. 5 is a circuit diagram of a conventional switching power supply known in the art. Referring to FIG. 5, the conventional switching power supply includes a rectifier circuit Rect 1, a transformer TR1, a capacitor C1 and a semiconductor switch Ql. The semiconductor switch is controlled by a conventional switching controller 510, such as a pulse width modulator. The rectifier circuit Rect 1 is a bridge rectifier circuit composed of diodes D1 through D4. The input of the rectifier circuit Rect 1 is connected to terminals 502, 504 which are connectable to an external source of AC voltage. The transformer TR1 includes a primary winding N1, a secondary winding N2 and a tertiary winding N3. The output of the rectifier Rect 1 is connected to the capacitor C1 via the tertiary winding N3 of the transformer TR1. A series circuit composed of the primary winding N1 of the transformer TR1 and the semiconductor switch Q1 is connected in parallel to the capacitor C1. A capacitor C2 is connected to the secondary winding N2 of the transformer TR1 via a diode D5. Output terminals 506, 508 are connected across capacitor C2 and provide points of connection for an external load (not shown).
By switching the semiconductor switch Q1 to an on state, the electric power stored in the capacitor C1 is transferred to the transformer TR1 through the path connecting the capacitor C1, the primary winding N1 of the transformer TR1 and the semiconductor switch Q1. Next, by switching the semiconductor switch Q1 to an off state, the electric power stored in the transformer TR1 is transferred to the capacitor C2 via the secondary winding N2 and the diode D5. When the semiconductor switch Q1 is switched off, a voltage is generated across the tertiary winding N3 that is N3/N2 (the winding ratio of the tertiary winding and the secondary winding) times as high as the voltage of the capacitor C2. When the sum of the input voltage Vac applied to terminals 502, 504 and the voltage VN3 of the tertiary winding N3 exceeds the voltage VC1 of the capacitor C1, a current flows from the AC power supply. By boosting the voltage of the tertiary winding N3 the period during which the input current flows is extended and, therefore, the power factor is improved.
FIG. 6(a) shows the input voltage and current waveforms of the switching power supply of FIG. 5 for an input voltage of 100 VAC. FIG. 6(b) shows the input voltage and current waveforms of the switching power supply of FIG. 5 for an input voltage of 200 VAC.
In the conventional switching power supply shown in FIG. 5, the increasing rate of the conduction period of the input current is determined by the ratio VN3/VC1, where VN3 is the voltage of the tertiary winding N3 and VC1 is the voltage of the capacitor C1. The voltage VN3, determined by the voltage obtained by multiplying the output voltage VC2 and the winding ratio N3/N2, does not change with respect to the input voltage Vac. The voltage VC1 is equal to the sum of the peak value of the voltage Vac and the voltage VN3 of the tertiary winding N3, i.e., VC1=SQRT(2).Vac+VN3. Therefore, the power factor is high as shown in FIG. 6(a) when the input voltage is 100 V. However, the power factor is lowered, as shown in FIG. 6(a), for the input voltage of 200 V.
If the voltage VN3 of the tertiary winding N3 is boosted to improve the power factor at the input voltage of 200 V, the peak value of the voltage VC1 of the capacitor C1 also rises. Generally, the typical breakdown voltage of an electrolytic capacitor is around 450 V. To withstand the boosted peak voltage of the capacitor C1, a plurality of electrolytic capacitors must be connected in series to reduce the voltage across each capacitor. However, such a series connection of multiple electrolytic capacitors enlarges the dimensions and increases the costs of the switching power supply.